The advent of GPS technology creates a new high-tech avenue for ocean mapping. This paper mainly discusses the application of GPS-RTK technologies in undersea topographic survey. Taking the project of Yangkou man-made island in Nantong city as an example, discuss the particular application of sea mapping in under-water topographic survey. The main job and work procedure of GPS technologies in under-water topographic survey are introduced. Problems need to be cared during the procedure and how to solve them also are discussed. Feasibility analysis of RTK technologies in undersea topographic survey is also done in this paper.